Final answer:
A flame test can identify an unknown element by observing the color emitted from the element in a flame, which correlates to its unique atomic emission spectrum.
Step-by-step explanation:
A flame test can identify an unknown element by observing the color emitted when the element is introduced to a flame. Different metal electrons emit different wavelengths of light when they return to their respective ground states, resulting in varied flame colors. These colors are part of the metal's unique atomic emission spectrum. So when we perform a flame test on an unknown substance and observe the color of the flame, we can compare that emission spectrum to known values for different elements to determine the content of the unknown substance.
It's also important to note that other observations, such as unexpected color changes, light given off, and gas production, are also indicators of chemical changes. However, for the specific purpose of identifying elements using a flame test, the emitted color is the characteristic used for identification.